In order to recover the transmitted digital signals from such conjugate carrier waves of like frequency identifying n different signal levels by the ratio of their peak amplitudes, it is known to demodulate these carriers with the aid of coherent detectors having control inputs connected to sources of two oscillations of carrier frequency in relative phase quadrature, these oscillations thus being of the form Kcos.omega..sub.c t and Ksin.omega..sub.c t where K is a constant and .omega..sub.c is the pulsatance of the carrier waves. With, say, oscillation Ksin.omega..sub.c t suitably synchronized with a carrier oscillator at the transmitting end to define a reference axis, the coherently demodulated carriers give rise to two input voltages defining respective coordinates of an imaginary coordinate system, referred to hereinafter as an orthogonal matrix locating a point on that matrix, which lies on a radius including with the reference axis a certain angle .theta. adapted to assume -- ideally -- any of n different values. In practice, this angle will vary on reception within certain tolerance limits about the n nominal values of .theta. .
These variations are due, at least in part, to two kinds of distortions occurring in such two-channel systems, namely an intrachannel distortion between signals transmitted in successive cycles and an interchannel distortion resulting from the interaction of substantially concurrently transmitted signals on the two channels. For a discussion of this general problem, in a somewhat different system using quadrature amplitude modulation (QAM) without mutual correlation, reference may be made to an article by D. D. Falconer and G. J. Foschini entitled "Theory of Minimum Means-Space-Error QAM Systems Employing Decision Feedback Equalization", Bell System Technical Journal, December, 1973, page 1821.
In commonly owned U.S. application Ser. No. 620,140, filed 6 Oct. 1975 by Giovanni Tamburelli, now U.S. Pat. No. 4,029,903, there has been disclosed a receiver for PSK digital signals with or without amplitude modulation wherein the aforementioned distortions are compensated by a feedback circuit which modifies the two input voltages delivered by the coherent channel detectors to a decision unit. That unit comprises an orthogonal matrix with two intersecting arrays of leads selectively energizable under the control of these two input voltages, the simultaneous energization of one lead from each array identifying one of n stages of a read-only memory as determined by the location of the point of intersection of the two leads in one of n sectors of the matrix. The data read out from any memory stage so addressed include an angle signal S.sub..theta., representing the signal level defined by the incoming carriers, as well as two collateral feedback signals Vsin.theta. and Vcos.theta. which are to be superimposed upon the input voltages detected during the next cycle.
The sectors of the matrix represent zones of optimal reception or of possible signal deviation within which a signal subject to normal distortions (due to thermal noise with Gaussian distribution) may stray upon reception. With simple angle modulation (i.e. in the absence of supplemental amplitude modulation) and with n = 8 according to the particular case described in U.S. Pat. No. 4,029,903, the sectors may be considered identical with a vertex angle of .pi./4, each sector being bisected by a radius including a respective angle .theta. with a reference line such as the abscissa axis of a system of cartesian coordinates. The geometries of the optimal zones, however, are more complex in systems with mixed angle and amplitude modulation. In the latter type of systems, moreover, the position of the zonal boundaries depends also on the coefficient of amplitude modulation; thus, a switchover from straight angle modulation to mixed modulation may produce different zone configurations, depending on whether the two systems are to operate with, say, equal peak power or equal average power.